As the integration degree of IC's increases with a rapid progress in electronic equipment, printed wiring boards have widely been used in recent years to meet requirements such as further enhancement in precision, density, and reliability.
The printed wiring boards are classified into a single-sided printed wiring board, a double-sided printed wiring board, a multi-layer printed wiring board, and a flexible printed wiring board. In particular, fields to which the multi-layer printed wiring board (hereinafter referred to as the “MLB”), wherein insulating layers are interposed between three or more electric conductor layers to integrate them, can be applied are extending since connection can be attained between any two of the conductor layers and between any one of the conductor layers and an electronic component to be mounted.
This MLB is formed by stacking one or more inner layer circuit plates and pre-pregs (made of epoxy resin or the like) alternately inside a pair of single-sided copper clad laminates or a pair of double-sided copper clad laminates as both-side exteriors, hot-pressing the resultant lamination, through cushion materials, by means of hot-pressing plates while nipping the lamination with a tool, so as to cure the pre-pregs and form an intensely-integrated lamination, making holes, subjecting the lamination to through-hole plating or the like, and etching the surfaces.
When such an MLB is produced, a release film is usually used between each of the copper clad laminates (exterior plates) and the tool. As this release film, a highly heat-resistant resin is used which is not melted in the heating and pressing step and is made of 4-methyl-1-pentene copolymer, polytetrafluoroethylene, acetate, polyester, polypropylene or some other polymer.
As the copper foil of the copper clad laminate, a copper foil having roughened surfaces is frequently used in order to heighten the adhesiveness between one of the surfaces and the epoxy resin. An example of this copper foil is the so-called black oxidized copper foil, which has surfaces roughened by oxidizing the surfaces or etching the surfaces with acid. In this case, the above-mentioned release film has a problem that the other of the copper foil surfaces gets into the surface of the film in the heating and pressing step because of insufficient rigidity of the film so that the film cannot be peeled.
Japanese Patent Application Laid-Open (JP-A) No. 3-73588 suggests that a film wherein a 4-methyl-1-pentene copolymer film is monoaxially drawn to improve the rigidity of the copolymer film is used as a release film. However, the 4-methyl-1-pentene copolymer film is insufficient in productivity and the draw ratio which can be attained, and also has problems about the external appearance of the film and the releasability thereof from a roughened surface of the copper foil, for example, the black oxidized copper foil surface.
JP-A No. 2002-225207 states that when a multi-layer film composed of 4-methyl-1-pentene copolymer and polyethylene or polypropylene is drawn, drawing unevenness and tear are less caused so as to yield a drawn film suitable for a release film, for producing an MLB, having high rigidity. However, if the hot pressing temperature is high at the time of the production of the MLB, the release film gets into the through-hole portions of the MLB so that the film deforms. Thus, this release film has a problem that the flatness of the film is not kept after the film is released.
JP-A No. 2003-1772 states that a multi-layer film composed of 4-methyl-1-pentene copolymer and polyethylene or polypropylene to which an inorganic filler is added is suitable as a release film, for producing an MLB, having high rigidity.
JP-A No. 6-134948 discloses a technique of a biaxial orientation multi-layer film wherein polypropylene resin layers are laminated on both surfaces of a central layer which has a specific thickness range and is made of 4-methyl-1-pentene resin containing heat resistant silicone, and a technique of peeling the polypropylene resin layers so as to produce polypropylene films. However, this technique is a technique for producing polypropylene thin films suitable for electrical condensers. Thus, this publication does not disclose any 4-methyl-1-pentene resin film having good releasability, which is suitable for release films. The biaxial orientation film made of the 4-methyl-1-pentene resin, which is obtained together with the polypropylene film by the present technique, cannot have a thermal coefficient of contraction of 20% or more, thin thickness unevenness and a good external appearance. Furthermore, the film is too weak in strength to be used as a release film since the film has a thin thickness. When the film is torn and broken, the film remains on the release surface. The film has problems about the releasability thereof from the roughened copper foil surface, for example, the black oxidized copper foil surface.